TLB Management via Status Bit to Skip IPIs
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Solution Overview
Problem
Current CPU architectures face inefficiencies in power management during Translation Lookaside Buffer (TLB) shootdowns, particularly in multicore systems, where unnecessary wake-ups from deep sleep states waste power and increase latency due to the serialization of TLB flush operations across cores.
Innovation Solution
Implementing a mechanism where cores entering low power states set a 'TLB clear' status in memory, allowing the operating system to skip sending Inter-Processor Interrupts (IPIs) to cores that have already invalidated their TLBs, thus avoiding redundant wake-ups and enabling cores to remain in sleep states until active again.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If the OS sends IPIs to all cores to invalidate TLB entries during shootdown operations, then TLB consistency is maintained across all cores, but power consumption increases and latency increases due to unnecessary wake-ups from low power states
Solution Approach 1:
The patent applies preliminary action by having cores proactively invalidate their own TLB entries and set the 'TLB clear' status bit in memory before entering low power states. This pre-action allows the OS to skip sending IPIs to these cores during shootdown operations, as their TLBs are already invalidated. The status bit serves as a pre-computed indicator that eliminates the need for subsequent wake-up communications, thereby reducing power consumption while maintaining TLB consistency.
2Reliability
If the OS sends IPIs to all cores to invalidate TLB entries during shootdown operations, then TLB consistency is maintained across all cores, but latency increases due to serialization of flush operations
Solution Approach 1:
The patent eliminates serialization latency by having cores perform TLB invalidation proactively before entering low power states. The 'TLB clear' status bit in memory serves as a pre-computed indicator that allows the OS to immediately identify which cores do not require IPIs. This eliminates the sequential wait-time for IPI acknowledgments from sleeping cores, as the OS can proceed with shootdown operations for only the active cores that actually need TLB flushing.
3Use of energy by moving object
If cores remain in low power states with invalidated TLBs, then power consumption is reduced, but system complexity increases due to the need for status tracking mechanisms
Solution Approach 1:
The patent uses a simple status bit copy mechanism where each core writes a single 'TLB clear' status bit to a designated memory location before entering low power states. This lightweight copy operation creates a persistent indicator that the OS can read without requiring complex tracking structures. The status bit serves as a simple boolean flag that eliminates the need for sophisticated state management, maintaining low complexity while enabling power-efficient TLB invalidation tracking.
Data Source
AI summary
An apparatus and method are described for reduced power TLB management. For example, one embodiment of a processor comprises: a plurality of cores; a first core of the plurality of cores comprising: a first translation lookaside buffer (TLB) to store address translations associated with page table walk operations, and power management logic to cause the first core to enter into a first low power state in which the address translations in the first TLB are no longer valid, wherein prior to entering into the low power state, the first core is to write an indication in a memory location that the first TLB no longer contains valid address translations; a second core of the plurality of cores to perform an operation requiring invalidation of one or more of the address translations previously stored in the first TLB, the second core to determine whether to transmit a request to the first core to invalidate the one or more address translations based on the indication.